A study of EMF mitigation underneath power lines and lightning-shielding enhancement by extra ground wires

This paper presents a theoretical study of the influence of extra ground wires (EGW) on both the electromagnetic fields (EMF) mitigation underneath 765 kV, 3-phase, double circuit transmission lines and the lightning-shielding performance. The method of moments (MOM) combined with the transmission-line model is used to simulate both the aforementioned transmission lines comprising the towers and the lightning channel in three dimensions, where the influence of the channel radiated EMF is taken into account. For power frequency AC energization, the EMF at 1 m above the ground level and at the surface of the phase conductors, and the induced currents in both normally used top ground wires and EGW are computed for various phase arrangements and numbers of EGW. In addition, the lightning-shielding performance with and without the EGW is introduced and discussed without AC energization. The induced voltages and currents in the phase conductors and ground wires as well as the voltage across the insulators are computed, with and without the EGW, due to different lightning-current amplitudes and waveforms, and return-stroke velocities. The most advantageous merits of using the EGW are as follows. (1) The EMF underneath the overhead power lines are drastically reduced with a negligible effect on the conductors' surface gradient and the charging currents. This can be generally used to meet regularity limits, especially when converting the existing 3-phase double circuit transmission lines to 6-phase systems so that the power handling capability can be increased by up to 73% via increasing the operating voltage. (2) The lightning-shielding performance is significantly improved by reducing the lightning-induced voltages and currents in the phase conductors as well as the voltage across the insulators, i.e. decreasing the probabilities of flashover and back-flashover and enhancing the reliability.